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#include <fstream>
#include <iomanip>
#include <cmath>
using namespace std;
ifstream f("dezastru.in");
ofstream g("dezastru.out");
int N, K;
double a[26][26], p[26], cmb=1.0;
int main()
{
f>>N>>K; a[0][0]=1;
for (int i=1; i<=N; ++i)
f>>p[i], a[i][0]=1;
for (int i=1; i<=N; ++i)
for (int j=1; j<=K; ++j)
a[i][j]=a[i-1][j]+a[i-1][j-1]*p[i];
for (int i=2; i<=N; ++i) cmb*=i;
for (int i=2; i<=K; ++i) cmb/=i;
for (int i=2; i<=N-K; ++i) cmb/=i;
g<<fixed<<setprecision(6)<<fabs(a[N][K]/cmb)<<'\n';
return 0;
}